Ice nucleating particles (INPs) induce freezing of cloud droplets at temperatures above their homogeneous freezing-point (approximately −38° C.) and at a relative humidity (RH) below the homogeneous freezing RH of aqueous solution droplets at lower temperatures. Accordingly, INPs influence cold cloud lifetime, phase, and their optical and microphysical properties. INPs are comprised of a diverse population of particles, some species of which have complex sources and sinks.
Developing a parameterization of INPs in global climate models (GCMs) that result in a credible representation of global cloud coverage and the radiative balance remains a challenge. In situ observations to close critical knowledge gaps, such as the vertical distribution of INPs in the air column, the complex sources and sinks of biological INPs, and INP influence on cloud microphysics, are identified as a high priority for the improvement of INP representation in GCMs. One of the largest biases in shortwave reflectivity exists over the Southern Ocean, and this bias may be influenced by poor representation of INPs over primarily oceanic regions. Measurements of INP number distributions, particularly in remote ocean regions are needed to help develop parameterization of ice nucleation for use in cloud-resolving models and GCMs. To further improve the parameterization of INPs in GCMs, both field and laboratory measurements are needed to identify drivers of ice nucleation in clouds. Accurately defining the activation temperature of INPs assists in understanding the influence of INPs on clouds and improving representation of INPs in GCMs because INP freezing temperatures influence cloud phase and lifetime in mixed-phase clouds, or the supersaturation or temperature conditions in which ice clouds can form. An accuracy of the INP concentrations applied in cloud and climate models to within a factor of ten can avoid biases that lead to significant differences in cloud radiative and microphysical properties.